The present invention generally relates to color gamut compression apparatuses and methods for converting a source color expressed by an information-input apparatus into a target color in a color gamut reproducible by an information-output apparatus and, more particularly, to a color gamut compression apparatus and method in which, when the information-input-apparatus and the information-output apparatus differ with respect to the color gamut, a first color outside the color gamut of the information-output apparatus is converted into a second color inside the color gamut of the information-output apparatus such that an original image is reproduced with as high fidelity as possible and high-brightness color and low-brightness color are converted into colors of sufficiently high chroma while maintaining high color consistency in the direction of brightness.
Information apparatuses such as displays, printers and scanners which process color image data have a range of color which may be input or output which is characteristic to the information apparatus. That is to say, such apparatuses have a color gamut. Color image signals may be transferred between information apparatuses of different types of color gamut for processing such that a color in the information-input apparatus is reproducible in the information-output apparatus provided that the color gamut of the information-output apparatus such as displays and printers includes the color gamut of the information-input apparatus such as scanners. If the color gamut of the information-output apparatus does not include the color gamut of the information-input apparatus, however, those colors that are inside the color gamut of the information-input apparatus but outside the color gamut of the information-output apparatus are not reproduced without undergoing a change in the information-output apparatus.
Thus, a color which is outside the color gamut of the information-output apparatus is output after conversion into a color inside a color gamut of the information-output apparatus. That is to say, when the color gamut of an information-output apparatus is not coextensive with the color gamut of an information-input apparatus, color gamut compression for converting a source color in the input-information apparatus into a target color inside the color gamut of the information-output apparatus is required.
One approach to the conventional color gamut compression method is disclosed in the copending PCT/JP98/01785 application yet to be published at the time of filing of the present invention. FIG. 8 illustrates a concept behind the related-art color gamut compression described in PCT/JP98/01785. More specifically, FIG. 8 shows color compression in a CIE/L*a*b* space, where L* indicates brightness and C indicates chroma. That is, a color along the L* axis is an achromatic color.
According to the related-art color gamut compression method of FIG. 8, a point of convergence is provided on the achromatic L* axis. A source color outside the color gamut of the information-output apparatus is converted into a target color on a point of intersection between a boundary of the color gamut of the information-output apparatus and a half line passing through the source color and ending at the point of convergence. Such a color gamut compression method is known to provide superior color consistency and ease of computation due to the fact that the point of convergence lies on the L* axis.
It is to be noted that, with respect to hue, there is discrepancy between the color space and the characteristic of human visual system. For example, areas of Cyan (hereinafter, indicated by C), Blue (hereinafter, indicated by B), Magenta (hereinafter, indicated by M), Red (hereinafter, indicated by R), Yellow (hereinafter, indicated by Y) and Green (hereinafter, indicated by G) with transition into one another in the stated order in the generally employed CLE/L*a*b* color space are characterized such that the hue areas of C and B are warped. For this reason, the related-art color gamut compression causes the area of B to intrude the area of C or M. The reproducible area of B is enlarged and those of C and M are reduced such that a color with a digital representation in the area of C or M is output as a color that contains a blue component, causing hue shift when the output color is observed.
Due to the discrepancy between the color space and the characteristic of human visual system, compression performed within the same hue may cause an image before compression to be visually different from an image after compression. Since the related-art color gamut compression using the CIE/L*a*b* color space compresses within the same hue, it is difficult to ensure satisfactorily high visual consistency with respect to hue.
Another disadvantage with the related-art color gamut compression is that, since the point of convergence lies on the achromatic L* axis, high-brightness colors and low-brightness colors tend to be compressed toward a color with low chroma, producing a relatively low-chroma image when observed.